Europe has dubbed 1985 the European Music Year, and travel officials are calling it the "biggest-ever musical celebration," a year-long extravaganza of music of all types throughout the continent: from rock to Rachmaninoff and beer hall to concert hall.

Two years in the planning, the celebration is a bonus for transatlantic travelers in 1985, since many of the presentations -- musical jamborees and massed spectacles -- will be free. Europe, of course, already has a busy calendar of annual music festivals, but a number of offerings -- including commemorative concerts in Cologne's recently renovated cathedrals -- will be one-time only events.

The festivities, which also include seminars and lectures, coincide with the 300th anniversary of the birthdays of composers Johann Sebastian Bach, George Frederick Handel and Domenico Scarlatti, all born in 1685. The works of these baroque masters will be prominent on concert programs.

The musical year gets underway in March with the International Bach Festival in Leipzig, East Germany, concentrated near the composer's birthdate of March 21. Bach lived and worked in Leipzig for much of his life.

Among the performing groups are the Academy of St. Martin-in-the-Fields and the Washington Bach Consort, the only U.S. organization invited.

Actually, the Bach festival is not an official European Music Year event, since it takes place in Eastern Europe, and the Music Year has been organized by 23 West European nations and Yugoslavia. But Bach's birthday must be included in any list of Europe's major musical happenings.

Among other special attractions are a museum exhibition in Madrid on "Scarlatti and His Era" from September to December and a major Handel festival in London from July 12 through 22.

One of the liveliest events of the year is expected to be a "day of music" on June 21, the beginning of summer. Performances are planned not only on many of the continent's famed stages but in the streets, parks and squares of towns, villages and cities throughout Europe.

France has a popular tradition of an annual day of music, but this is the first time all of Europe will join in. In Ireland, for example, bands from all over the country will join in a grand open-air concert in Dublin.

The year's list of presentations is lengthy, since something is happening almost every day. For detailed information:

* A calendar of major events of the European Music Year (to date; it's still growing) can be obtained from the European Travel Commission, 630 Fifth Ave., Suite 610, New York City, N.Y. 10111.

* The musical events in a specific country can be obtained by contacting that country's national tourism office in New York City. These offices also should be able to advise on how to arrange for tickets, and they can provide the names of any tour organizers offering music-related tours.

Among the music tours available to Europe:

* "Understanding Bach": A series of three packages to the Leipzig Bach festival, designed by Odyssey Tours Inc. of Santa Monica, Calif. Each package varies in which of 14 medieval and baroque cities of East and West Germany and Czechoslovakia are visited, but all three come together for a week at the Bach festival, March 19-27.

During that week, participants will have tickets to six major performances and are invited to attend seminars by visiting artists and to join tours to sites identified with Bach's life and work.

Leaders on the Leipzig packages are American concert opera singer Mordecai Bauman and Michael Rubinovitz, who produces audio-visual materials for the Metropolitan Opera Guild.

Tours depart March 11 for 22 days ($3,195 per person double occupancy for air fare, lodging, two meals daily); March 15 for 22 days ($2,695); and March 15 for 18 days ($2,395). For more information: Odyssey Tours Inc., 1831 Wilshire Blvd., Suite 120, Santa Monica, Calif. 90403, (800) 654-7975.

* A Day at the Bach Festival: Dailey-Thorp Travel of New York, which specializes in music tours, has scheduled one week in Dresden, East Germany, for the opening of the rebuilt Semper Opera House. A one-day (and night) side trip, however, is scheduled to the Bach festival in Leipzig for a performance of Bach's "St. John's Passion."

The tour departs March 21, and the cost is $1,753 per person double occupancy for lodging, meals and excursions (air fare extra). For information: Dailey-Thorp Travel, 315 West 57th St., New York City, N.Y. 10019, (212) 307-1555.

* ASPEN TRAILS: The Aspen/Snowmass area in Colorado offers some of best downhill skiing in the country, but the two Rocky Mountain resorts also are developing into a major ski-touring center. A few weeks ago, the new five-mile Oak Creek Trail linking Snowmass and the Buttermilk Mountain area of Aspen was officially opened.

The trail will be maintained by the newly formed Aspen Nordic Council, headed by former U.S. Olympic cross-country skier Craig Ward, who grew up in Aspen. The organization is promoting the expansion of cross-country skiing in the Aspen area.

Ward says that while touring with the ski team for eight years, he often found excellent cross-country facilities at major downhill resorts in Europe.

In the United States, "we're about 15 to 20 years behind Europe" in what's available to cross-country skiers, he says. But he hopes to pattern the Aspen trail system after "a European-style resort."

The Oak Creek Trail, which winds alongside Oak Creek, is part of a 20-mile network of regularly groomed trails in Aspen now open to the public at no charge. It's a fairly easy trail, says Ward, which skiers can tackle on their own without a guide. They can pack a lunch, take their time en route and catch a shuttle bus back to their departure point.

By next year, the council hopes to have 50 miles of linked trails available, some into the wilderness and others between business and residential areas.

Within the Aspen area are trails for all levels of experience, from the very easy Rio Grande Trail along the old railroad bed paralleling the Roaring Fork River to back-country hut-to-hut skiing at 10,000 feet in elevation and higher.

Several ski-touring schools offer lessons for skiers of all abilities, from beginning to expert.

For more information: Aspen Nordic Council, Box 10815, Aspen, Colo. 81612, (303) 925-4790.

* INNER MONGOLIA: Here's a tour for the adventurous -- by horseback and camel to the high grasslands of Inner Mongolia, living (sort of) the life of a Mongol herdsman. There is an appeal to seeing the world's remote places that makes the hardships worth it.

The 22-day trip, departing June 27, is offered by Boojum Wilderness Institute of Leucadia, Calif., which specializes in outdoor educational programs for Southern California schools for most of the year and organizes off-beat trips in the summer.

The package includes a three-day visit to Peking and stops at two cities on the four-day train journey north to Hohhot, the capital of Inner Mongolia. But eight days will be spent on the Mongolian trek on a loop ride from Hohhot through the grasslands. Participants will either pitch camp or stay in Mongolian yurts at communes in the countryside.

The land cost, including meals, lodging and mounts, is $2,300 per person. Round-trip air fare between San Francisco and Peking is another $1,200. The group will be limited to about 18 participants.

This particular trip is a first for Boojum and for Chinese travel officials, so the institute warns there could be unexpected changes in the itinerary. "It's exploratory," says director Kent Madin, "and that appeals to us a lot. There's a fresh nature to it." Boojum led a mountain backpacking trip to China last summer and is repeating it this year.

Facilities will be "the best available," says Boojum, "which in Mongolia may mean tea with mare's milk for breakfast and hot water to bathe every other day."

For more information: Boojum Wilderness Institute-China Expeditions, Box 2236, Leucadia, Calif. 92024, (619) 436-3927.

* GOURMET AUSTRIA: There will be dining of a different sort on this series of two-week "deluxe gourmet" tours to the best restaurants of Vienna, Salzburg and Innsbruck.

A Viennese "feast" at the famed Sacher Hotel will welcome participants with a menu that may begin with soup with fluffy dumplings and pate' and conclude with the Sacher's famous whipped-cream-topped chocolate torte, cream cheese dumplings with plum sauce and rich Viennese coffee.

On another evening, following the opera, dinner is at Vienna's oldest restaurant, Zu den Drei Husaren, which serves a classic venison "as you have never tasted before," says Austrian-born Annemarie Victory of New York, a New York travel executive who specializes in gourmet tours. "Austria has the best venison in Europe."

One night is spent in Lichtenstein and three in Switzerland, where a lunch in Crissier is planned at the restaurant of Fredy Girardet, described by some food critics as the world's greatest chef. Girardet will choose the menu and pick the proper wines to accompany it, says Victory.

The price is $2,890 per person (double occupancy), which includes lodging, meals (with wine) and motor-coach travel. Air fare is extra. Departures May 31, June 12 and Sept. 13.